Resumen de The use of omic-technologies in the assessment of heat-stressed lactating dairy goats
Heat stress (HS) causes significant losses in the dairy goat industry when temperature- humidity index (THI) is >75. ‘Omic’ technologies offer a holistic approach to figure out how goats cope with HS and to find biomarkers. With this aim, 3 experiments (Exp.) were carried out using Murciano-Granadina dairy goats and a climatic chamber with metabolic boxes. Lactating does (n = 32) were fed a total mixed ration, freely watered and milked ⨯1 daily under different climatic conditions. They were: TN (thermal neutral, THI = 59-65) and HS (day, THI = 86; night, THI = 77). Photoperiod (light- dark) was constant (12-12 h). Physiological and performance traits were recorded daily, milk composition sampled weekly and BW at the start and the end of each Exp. period. In Exp.1, changes in blood transcriptome of 2 groups of 4 does (n = 8), under TN or HS were studied for 35 d. In addition to performance impairment, microarrays of blood samples at d 35, revealed that HS up-regulated 55 genes and down-regulated 88. Dynamic Impact Approach analysis showed 31 biological pathways affected by HS. Effects were negative in these related with leukocyte transendothelial migration, cell adhesion, hematopoietic cell lineage, Ca and PPAR signaling, whereas were positive on those activating nucleotide metabolism. In conclusion, HS worsened milk performances and altered the functionality of immune cells, which may result in a less competent immune system for fending-off diseases. In Exp.2, HS candidate biomarkers in urine were assessed by 1H NMR (proton Nuclear Magnetic Resonance)-based metabolomics. Does (n = 16) were submitted to the TN and HS conditions in a crossover design lasting 35 d. Partial least square-discriminant analysis with cross validation were used to separate between TN and HS clusters. Discriminating metabolites were Phenilalanine (Phe) derivative toxic compounds (OH-phenylacetate, OH-phenylacetylglycine, phenylglyoxylate and hippurate), which increased in HS vs. TN does. Increased urinary excretion of these compounds indicated a harmful gastrointestinal microbiota overgrowth by HS, which sequestrated dietary aromatic amino acids. Consequently, HS does should have decreased the synthesis of neurotransmitters and thyroid hormones, impairing milk yield and composition. In conclusion, lactational impairment of HS does was reflected in their metabolome by the presence of gut-derived toxic compounds in urine. Phe derivatives and hippurate were identified as key urinary biomarkers of HS dairy goats. In Exp.3, lactating dairy goats (n = 8) were submitted to the TN and HS conditions for 15 d and milk candidate biomarkers assessed by 1H NMR. On d 12, does were challenged with E. coli lipopolysaccharide (LPS) or saline (CON) by udder-half and milk samples collected post-challenge (h 0, 4, 6, 12 and 24). Treatments were: TN (CON and LPS) and HS (CON and LPS). Milk citrate increased in HS revealing a shift in macrophages’ function (i.e., transporting mitochondrial citrate to cytosol to produce inflammatory mediators). Differences between TN and HS in response to LPS over time where observed by PLS-DA. Milk metabolome in TN-LPS udder halves was less affected and restored earlier than in HS-LPS halves. Most discriminating metabolites were choline, N-acetylcarbohydrates, L-lactate, -hydroxybutyrate (BHBA) and phosphocholine. Overall, milk metabolomic profiles were markedly affected by ambient and udder health conditions. Citrate and choline indicated the occurrence of oxidative and inflammatory stages in HS and LPS stressed mammary glands, respectively, and were proposed as key biomarkers in milk.
